Alright, I’m trying to bring automation to my pool pump but it’s a little trickier than I’ve seen others discuss. I have the standard 3-wire setup (two 120v mains and a ground) running through the tried and true Intermatic timer. The timer is just on/off. One 120v wire runs to the motor. The second 120v though runs through a SPDT switch that acts as the high/low selector switch (For those unaware or if I’m unaware, I mean this switch cannot be off, it has one input and two outputs with one output always active and both cannot be active at the same time.) Is there a failsafe way to recreate this circuit?
I have no idea what happens if you energize hi&low simultaneously and would like to make sure it never happens. (I used to think “what are the odds?” until I created a race condition in a ST lighting automation that didn’t trigger for years - until we went on vacation and a neighbor triggered it. A couple of hundred dollars in lights died that holiday after they strobed non stop for somewhere over 48hrs.) 
Thanks in advance and if this has been covered, I do apologize. It wasn’t for a lack of searching.